internal: prepare Zed worktrees automatically

[samohovets]

Summary

• Add a Zed create_worktree task that prepares new linked worktrees without taking editor focus.
• Reuse one setup script for frozen dependency installation and the Turbo build.
• Keep failed setup output visible while hiding successful runs.
• Closes Prepare new Zed worktrees with cached install and build tasks #6.

Changelog

Public summary:

• None.

Context:

• New Zed worktrees now reuse pnpm's shared store and Turborepo's built-in linked-worktree cache behavior instead of starting from an uncached checkout.

Verification

• ./scripts/setup-worktree.sh
• Repeated setup confirmed full Turbo cache hits.
• Concurrent setup completed in two linked worktrees with spaces in their paths.
• pnpm fmt:check
• pnpm validate
• jq empty .zed/tasks.json
• shellcheck scripts/setup-worktree.sh